Floating Voltage Problem

[AOzturk]

Hi,

We have recently purchased an NI PXI system with an X series DAQ card (6363). PXI system is controlled by a laptop via an express card (no onboard computer). 

I have some troubles measuring the analog voltage output of a torque sensor. 

The torque sensor has five connections:

1. Supply

2. Supply GND

3. Torque Voltage Output

4. Torque Voltage Output GND

5. Shield

The problem is as follows.

When I measure the voltage between lines 3 and 4 using a voltmeter, I read a constant value of 0.051 V. When I connect the same lines to the DAQ device, the value floats between 0.02 V and 0.08V if RSE is used.

I have also tried differential setup. I connected AI - to AIGND. I have also tried connecting resistors ( 10 K to 100K) between AI-,+ and AIGND. Although some improvement is observed, the values are still floating. The strange thing is that the value read from the voltmeter is always constant.

The vi is simple. I have just used DAQ assistant. Single channel. Screenshot is attached.

I was using a PLC before which also provides non-floating value for this sensor.

Any help would be greatly appreciated. 

[Henrik_Volkers]

Could that 'floating' be a result of line noise (Hum) sampled at 100Hz ? 

If you sample 1000 points at 1kS/s (or 10kS/s), how does your signal look like?

(If you have an analog scope, how does it look like?)

If you do a AC measurement with your voltmeter, what does it show (60mV??)?

How fast is the signal of your torque-sensor changing? 

Your stable signals are likely a result of a lowpass filter < 50Hz ;).